This invention relates to an improved mechanical shaft joint boot (hereinafter simply referred to as a "boot").
In this specification, the term "crest" indicates the top portions of individual ridge portions in a bellow portion.
In the field of boots being attached, for example, to a constant velocity joint of automobiles and the like, hitherto, a boot 1 as shown in FIG. 1 was known which is made up of a large-diameter ring portion 3, a straight portion 9, a bellows portion 7 whose first bottom portion (as numbered from the side of the large-diameter ring portion 3, this manner of numbering being also applied to crests) is connected with the straight portion 9, and a small-diameter ring portion 5 in that order. In the arrangement above, the purpose of providing the straight portion 9 between the large-diameter ring portion 3 and the bellows portion 7 is to prevent the first bottom portion 13 from being caught in a joint 2 shown in FIG. 2 and to avoid interference with other members inside a narrow engine room, i.e. to make a so-called whirlingness good (see Japanese Utility Model Applications Laid-Open Nos. 58-142455 and 58-178562).
Further, due to the presence of the straight portion 9, the distance from the center of articulation of the joint 2 to the deformable portion (the bellows portion 7) of the boot 1 becomes large. Accordingly, to make the boot 1 most adaptable to an inclination of a shaft 24, it is general practice to make the diameter of a first crest portion large, but within the limits of whirlingness.
In assembling the boot 1 to the joint 2, the large-diameter ring portion 3 is fitted on a large-diameter ring mounting portion 23 of a casing 21, the small-diameter ring portion 5 is fitted on a small-diameter ring mounting portion 25 of the shaft 24, and these portions are tightened with clamps or the like. In the foregoing arrangement, the relationship: l1.gtoreq.l2 is generally held with respect to the distance l1 between the large-diameter ring portion 3 and the small-diameter ring portion 5 and to the distance l2 between the large-diameter ring mounting portion 23 and the small-diameter ring mounting portion 25. In this specification, of course, l1 represents the distance between the large-diameter ring portion 3 and the small-diameter ring portion 5 when the boot is in a free or no-load state. If a peculiar relationship: l1&gt;l2 is selected for the two distance l1 and l2, this means that the boot previously held in compression be attached to the joint 2 for purpose of promoting expansion of the film of the bellows portion 7 within the definite distance between bellows the straight portion 9 and the small-diameter ring portion 5.
As described above, however, the distance from the center of articulation of the joint 2 to the deformable portion (the bellows portion 7) of the boot 1 becomes large because of the presence of the straight portion 9. Therefore, when the shaft 24 is made inclined, the defect necessarily occurs that the boot 1 is severely compressed.
In this respect, it has become clear through the examination done by the present inventor that wear is hastened at the first crest 15 (the vicinity of its top portion) coming into contact with a shoulder portion 11 to the straight portion 9, as shown in FIG. 3, thereby deteriorating the durability (see test results hereinafter described). It has also become clear that the durability of the first crest 15 governs the overall durability of the boot 1.